J . A . Corbacho et al .: Radioprotection 2024 , 59 ( 2 ), 80 – 87 81
focused on mapping an area with significant U concentrations , using a mini-airborne equipment based on two BGO detectors . In addition , the study by Ji et al . ( 2019 ) focused on the use of LaBr 3 ( Ce ) in airborne gamma-ray spectrometry using a drone to assess dose rates at diverse flight altitudes but did not provide information about the quantification of radionuclide concentration in the soil . To solve this lack of information , it is also important to highlight the recent work carried out by Rusnák et al . ( 2023 ) within the framework of the European Metrology Program for Innovation and Research ( EMPIR ), where they describe the development of a high-capacity ( autonomy and payload ) UAV equipped with a High-Purity Germanium ( HPGe ) detector .
Depending on the goal for which the UAV is to be used , it is necessary to find a balance between the capacity of the UAV , the type of radiation detector and the desired sensitivity and accuracy of the measurement . It is not possible to develop a generic UAV for all types of missions . One of the objectives of this work is to develop a rotary-wing UAV focused on the identification and quantification of both point and extended sources . With regard to extended sources , the purpose of the UAV developed in this work is to characterise areas of radiological anomalies in order to produce radiological maps of the environmental equivalent dose rate and to quantify the activity of the radionuclides present in specific places . All of this is framed within the framework of the actions of a radiological emergency .
2 Material and methods
2.1 Description of the rotatory-wing UAV
Using rotatory-wing UAVs ( hereinafter drone ) to accurately locate , identify and characterise radioactive sources in near-real time is a twofold challenge . To further optimise drone-based radiation sensing systems for another application , it is necessary to combine the appropriate drone and sensor components by understanding their nature , physics and advantages and disadvantages in the context of a mission ’ s goals .
The drone selected for the study is an octocopter FPV8 . Its main characteristics are : an average flight time of 20 – 30 min , automatic take-off and landing , automatic position maintenance , return home on signal loss , automatic route programming via GPS , an integrated camera with near real-time image recording and transmission , and a load capacity of 5 kg .
Depending on the type of measurement to be conducted , the detectors should be chosen carefully . In general , scintillation detectors with low resolution , such as CsI ( Tl ), are most suitable for detecting orphan sources , as detailed in Baeza et al . ( 2018 ). However , when accurate quantification of the activity of a radioactive source , whether it is an extended source or a point source , is required , LaBr 3 ( Ce ) or NaI ( Tl ) detectors are more suitable . For this study , a LaBr 3 ( Ce ) detector with a volume of 43.3 cm 3 has been selected .
Each radiological measurement is associated with a GPS position provided by a GPS receiver . The management of the different devices installed on the drone and the data they provide are carried out by home-made software applications developed in different programming languages ( Python , C ++ and Wiring ).
To ensure effective operation of the designed system with in situ and real-time detection , communication between the drone , its instruments , the operator , and decision-making centres is crucial . Two communication channels have been implemented , selected based on the radio-electrical environment during flight : a ) Internet communication via 4G USB modem . b ) Communication via LoRaWAN TM technology , utilising 868 or 900 MHz frequency bands . This enables efficient communication while keeping the payload low .
2.2 Efficiency calibration
In order to calculate the activity of a radioactive source , it is essential to know the detection efficiency of the gamma detector used . The efficiency calibration of two source geometries has been implemented : a point source and an extensive homogeneous plane source .
In the case of a point source , the detection efficiency , e , was determined experimentally by measuring different radioactive point sources ( 152 Eu , 137 Cs and 241 Am ) of known activity placed at a distance of m from the detector , coinciding with its axis . The value of the efficiency is obtained by fitting the experimental measurements to the expression :
e ¼
N A source pðEÞ ;
where N is the area gross counts of the photopeak of interest per unit time recorded at the detector ( cps ) for each source , A source is the decay corrected source activity , and p ( E ) is the emission probability for the measured photopeak .
Assuming that the geometry of the source can be assimilated to a flat and homogeneous infinite surface , the detection efficiency is determined semi-empirically . The components involved in this calculation are the gross count for a given energy , the photon flux reaching the detector and the activity of the source , as proposed by Beck et al . ( 1972 ) and detailed in ICRU report n ° 53 ( ICRU , 1994 ) i . e .:
N ¼ N
N0 f ; ð2Þ
A x N 0 f A x
Where N A x are the gross counts registered under the photopeak for each energy E i , depending on the concentration existing in the source , i . e ., the detection efficiency . N N 0 is the detector ’ s angular correction factor for the energy E i . N 0 f
is the count per unit of total uncollimated flux of a parallel beam of photons of energy E i incident normally on the surface of the detector . Finally , is the primary flux of gamma photons of energy E i emitted depending on the activity present in the radioactive source , expressed in ( g cm �2 s �1 Bq �1 g ). The value of this term is tabulated in the ICRU report 53 and depends on the depth distribution of the radionuclides .
Concerning the calculation of the minimum detection activity ( MDA ), is computed as Nir-El and Haquin 2001 indicate for in situ gamma spectrometry . In situ measurements using germanium or scintillation detectors have been extensively tested and are widely cited in the literature .
f
A x ð1Þ